1. Field of the Invention
The present invention relates to a semiconductor device provided with a circuit including a semiconductor element such as a transistor, and a method for manufacturing the semiconductor device. For example, the present invention relates to a power device which is mounted on a power supply circuit; a semiconductor integrated circuit including a memory, a thyristor, a converter, an image sensor, or the like; an electro-optical device typified by a liquid crystal display device; a light-emitting display device including a light-emitting element; and an electronic device on which any of the above is mounted as a component.
In this specification, a “semiconductor device” generally refers to a device which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all included in the category of the semiconductor device.
2. Description of the Related Art
Transistors formed over a glass substrate or the like are typically formed using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors formed using amorphous silicon have low field-effect mobility, they can be formed over a larger glass substrate. On the other hand, although transistors formed using polycrystalline silicon have high field-effect mobility, they are not suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique in which a transistor is formed using an oxide semiconductor and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is formed using an In—Ga—Zn-based oxide as an oxide semiconductor and such a transistor is used as a switching element or the like of a pixel in a display device.